A phase transition has been observed in the system pyridine adsorbed on Ag(lll) at 100 K by near-edge x-ray-absorption fine-structure measurements in real time. At low pyridine coverages an angle between the ring plane and the surface plane of 45° ±5° was observed. This phase converts sharply at a submonolayer coverage to a phase with an angle between the ring plane and the surface plane of 70° ± 5°. Continued exposure gradually leads to a randomly oriented multilayer.PACS numbers: 68.35.Rh, 68.35.Bs, 78.70.Drn There is widespread interest in the pyridine-silver system because of its importance for surface-enhanced Raman scattering. 1 " 3 In particular, some proposed enhancement models involve charge-transfer excitations from the metal to affinity levels of the adsorbate, 4,5 which are also probed by the near-edge x-rayabsorption fine-structure (NEXAFS) technique. 6 " 8 Pyridine (C5H5N) is electronically characterized by a nitrogen lone-pair orbital. Pyridine chemisorption on metal surfaces may therefore have model character for determining the relative contributions of lone-pair and TT bonding to the chemisorption bond. Here we report the coverage-dependent molecular orientation of pyridine chemisorbed on Ag(lll) at 100 K determined by NEXAFS.Demuth, Christmann, and Sanda 9 studied the chemisorption of pyridine on clean Ag(lll) surfaces at --140 K with vibrational electron-energy-loss (VEELS) and uv-photoemission spectroscopies (UPS). They observed a phase transition at about half a monolayer coverage from a nearly flat-lying 7r-bonded pyridine phase to an inclined N-bonded phase. Similar coverage-dependent orientational phase transitions have been obtained by VEELS for pyridine on Ni(001) 10 and Pt(llO). 11 Orientational phase transitions also occur as a function of temperature. This has been shown for pyridine on Ni(001) by VEELS 10 and for pyridine on Pt(lll) in a recent NEXAFS study. 12 The NEXAFS measurements on Pt(lll) indicated a low-temperature pyridine state with an apparent angle between the ring plane and the surface plane of 52° which converts at 7^300 K to a high-temperature state with a corresponding tilt angle of 74°. This result, as well as UPS and electronic EELS data for pyridine on Ag(lll), 1314 casts some doubt on the existence of flat-lying pyridine molecules on well-defined surfaces at low coverages.NEXAFS studies on molecules which are only weak-ly perturbed by chemisorption are particularly useful in determining the orientation of the molecules relative to the surface. 6 " 8 For low-symmetry surfaces this includes the determination of the azimuthal orientation. 15 The polarization dependence of NEXAFS transitions is due to the validity of dipole selection rules for photoabsorption. The analysis of NEXAFS is especially unambiguous when TT resonances occur resulting from transitions of a Is electron into unfilled antibonding TT states. These TT resonances are rather sharp compared with a-shape resonances so that background subtraction is straightforward. With high-brightness storage ring...

We report the structural and electrical characterization of thin films of organic semiconductor molecules consisting of an oligothiophene core capped with electron-withdrawing tricyanovinyl (TCV) groups. X-ray diffraction and atomic force microscopy of evaporated films of three different TCV-capped oligothiophenes showed that the films were highly crystalline. Electrical transport was measured in thin film transistors employing silver source and drain contacts and channel probes to correct for contact resistance. Three compounds exhibited n-channel (electron) conduction consistent with cyclic voltametry data that indicated they undergo facile reduction. Maximum electron mobilities were 0.02 cm2/V.s with an on/off current ratio of 10(6). A fourth end-capped molecule, TCV-6T-TCV, which had six thiophene rings, exhibited both p- and n-channel transport. Overall, these results confirm that substitution of oligothiophene cores with electron-withdrawing groups is a useful strategy to achieve electron-transporting materials.

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Near edge X-ray absorption fine structure (NEXAFS) has been used to establish the orientation of the six-membered ring molecules benzene, pyridine, pyrazine and s-triazine adsorbed on Cu and Ag single crystal surfaces. For submonolayer coverages of pyridine on Ag(111) an orientational phase transition has been observed with NEXAFS in real time

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A series of three dicyanovinyl (DCV)-substituted oligothiophenes (referred to collectively as DCV-nT-DCV, where n = 1-3) were prepared and their electrochemical and structural properties were examined. The introduction of this group results in favorable structural features: planarity, π-stack formation, and close intermolecular interactions as revealed from X-ray single crystal analysis. Electrochemical measurements suggest that these molecules are easy to both oxidize and reduce and thus are expected to show ambipolar charge transport properties similar to those we reported earlier on a closely related series of oligthiophenes endowed with the tricyanovinyl group (TCV).